This invention relates to an apparatus utilizing infrared photo detection and associated electronic circuitry to determine the respective times when a ballistic projectile traverses through a focused infrared light curtain to calculate instantaneous speed, and thus, ballistic velocity.
In measuring ballistic projectile velocity, it has been common practice to use a pair of broad-band photodetectors spaced apart at a fixed known distance to produce, in turn, a signal to a common timing circuit at the point in time that a projectile interrupts the light detected by the photodetector indicating the projectile has just passed over each detector. The interval between each of the signals from the photodetectors indicating that a projectile has been detected and the known distance between the pair of photodetectors are used to compute the velocity of the ballistic projectile as it traversed the course, i.e. the distance, between the photodetector assemblies.
The assemblies housing the photodetectors, which are typically photo diodes or phototransistors, normally operate by having these receiving elements placed so that they can receive steady broad-band illumination from a constant light source such as the sun, specialty high intensity incandescent bulbs, or other broad-band light source. The photodetector assemblies are typically called xe2x80x9cscreenxe2x80x9d in the ballistic projectile velocity measurement field for the reason that such assemblies have replaced earlier physical conducting grids or screens which recorded the passage of a projectile. Earlier screens were designed to utilize either incandescent light sources or ambient light from environmental sources, i.e. the sun. Screens utilizing ambient light from the sky required an optical slit, or other masking device, to restrict extraneous sources of light from being detected by the photodetectors.
With early photodetector assemblies, one typical problem encountered was that the illumination received by the photodetection receivers was not constant because of varying photoemission levels from the incandescent lamp or lamps, a varying amount of dust or other particles in the light transmission path between the source and the receiver, aging of the incandescent lamp filament, varying distances from the light source and the receiver, and varying atmosphere conditions and angles of the light source, i.e. the sun, in the case of sky screens. Particularly with regard to sky screens, the photosensitive receivers view only a segment of unobstructed sky through an optical slit which created a problem with the illumination level from the light source. On clear days, the illumination level was less than on hazy or partially obscured (cloudy) days and, for that reason, in order for a sky screen to properly operate under available light conditions, the light needed to be diffused, rather than be received directly from the source. Thus, while the optical slit created a partial focusing of the light, the diffuser element dispersed the direct light over a broader expanse, which also reduced potential problems from reflected light. Neither light source, incandescent or ambient, produced a constant, fixed level illumination source of light from which the photodetection receivers could detect the passage of a projectile because of the number of variables existing in the illumination source and from environmental conditions.
One environmental problem referenced above, i.e. reflected light, can cause a number of different types of misreadings by the photodetection receiver. When using ambient light from the sun, directed through an optical view slit (but without a diffuser), it was possible that the photodetector would respond because the circuit associated with that receiver was designed to detect any perturbation in the light level about the ambient level, regardless of the direction of that perturbation, lighter or darker. This is to say that reflected light from the projectile can be less than, approximately equal to or greater than the amount of diffused light blocked by that projectile. If the reflected light is significantly greater than or significantly less than the amount of diffused light blocked by the projectile, the photodetection receiver would react as described above. In the case in which the reflected light is approximately equal to the amount of the diffused light which reaches the photosensitive element in the absence of a projectile, the receiver sees no significant change in the light level even though the projectile is passing between the light source and the receiver. Thus, a shadow, which would be expected to fall across the photosensitive element or elements, is essentially obliterated by the reflected light. Due to the cancellation of the shadow, which occurs earlier then the calculation within the apparatus where a change in light level is converted to an electrical signal and a time tag is placed on the signal, there is no way to recover the lost information by using any substituted electrical signal processing technique.
Attempts to overcome the problems of insufficient light to cause a shadow to interrupt the light being detected by the photodetection receiver and from reflected light which causes inaccurate and/or incorrect detection have required apparatus measuring projectile velocity to include focusing arrays to direct light, which would otherwise disperse, to not disperse as quickly so that a greater level of illumination is presented to the photodetection receiver when the light source is ambient environmental light, i.e. the sun. The problem and the cure are described in U.S. Pat. No. 4,239,962 [Oehler] for xe2x80x9cSun Shield and Light Diffuserxe2x80x9d. This patent describes a sun shield for redirecting ambient light away from the photodetector/sensor so that only a small amount enters through an opening directly above the sensor and is then diffused to flood the area of the sensor with a light having substantially the same light intensity so that errors in sensing the passage of a projectile through low level light intensity and reflected xe2x80x9cshadowsxe2x80x9d are substantially eliminated.
While it appears that the Oehler patent is the final word on using incandescent light or sunlight as the source of illumination for photodetection sensors or receivers, a different type of light, one which is not broad-band, was attempted. In U.S. Pat. No. 4,180,726 [De""Crescent] for xe2x80x9cA System for Measuring Characteristics of an Object in Motionxe2x80x9d, the apparatus described uses pulsed infrared light beams to detect the passage of an object between the light beam source and the detector. Relatively low speed motion of balls involved in sports, sports apparatus swings, etc., are discussed but there is no mention of high speed or high velocity projectile measurement. This is largely due to the pulsing of the infrared beam which may be at the precise instant that the projectile traverses the path of the beam, when the beam is not present, and the detector will be unable to sense an interruption in the light beam.
Therefore, it is an object of the present invention to provide a ballistic projectile velocity measuring apparatus which has a more stable and constant light source for providing sufficient light intensity across the entire area within the screen or frame in order to be able to unfailingly detect the passage or traversal of the projectile through the screen as it impinges upon or traverses the light beam.
It is also a further object of the present invention to provide a contiguous series of light sources within the screen or frame to produce a special wavelength light beam emanating from above and traversing the internal area of the screen or frame downward to a photodetector/sensor array with the lightwave length selected so that the possibility of reflected light causing a sensing error is minimized.
It is still a further object of the present invention, in order to provide a more uniform illumination throughout the entire area within the screen of frame, to create a light curtain formed by multiple light sources of the special wavelength and the photodetector array which permits the detection of the passage or traversal of the projectile at any point through the light curtain.
It is also an object of the present invention to provide an apparatus to be linked to the screens or frames to receive the sensor information, time mark or tag that information, calculate the speed or velocity of the ballistic projectile, store such speed or velocity of the ballistic projectile in non-volatile memory and be able to control a display and recall all data and information regarding the time sensor data for the ballistic projectiles.
It is still a further object of the present invention to be able to provide a printed report of the time sensor data of the ballistic projectiles and to transfer information from the display and calculation apparatus to a multi-purpose computer and/or print reports of the time sensor data.
Other objects will appear hereinafter.
A ballistic projectile velocity measurement apparatus is described which includes two screens which are spaced apart at a known or fixed distance for providing time measurement start and stop signals indicative of the passage of a ballistic projectile through a light curtain formed by contiguous focused light beams from plural infrared light sources. The passage of a ballistic projectile through the light curtain will cause a perturbation or fluctuation in the light intensity level received by one or more photo-optic sensors. The detection and time measurement is used to calculate the speed or velocity of the ballistic projectile. A calculation and display unit for receiving the time measurement signals, calculating the speed or velocity of the ballistic projectile and storing and displaying the sensed and calculated information is connected to the screens and controlled by keyboard command is also described.
The invention may be best described as a measurement and calculation apparatus for measuring the time a ballistic projectile enters and exits a prescribed course, passing through preselected zones along the course. The apparatus includes first and second screens having sidewalls angled outward from a base and capped by a light shield. These screens are arranged in a sequential array downrange from a firearm shooting position and are positioned laterally across the intended shot line of a ballistic projectile. Infra-red illumination means mounted below the light shield produce a light beam or curtain extending downward into and across substantially the entire area within each screen impinging upon an elongated rectangular slit in the top side each base. A plurality of photodetector sensors contained in each base below the rectangular slit are responsive to perturbations in the infra-red light made by the passage of a ballistic projectile through the light beam or curtain and, following detection of the light perturbation, generate timing signals indicative of the passage of the ballistic projectile. A calculation and display means is responsive to the timing signals from the photodetector sensors in the first and second screens for measuring the time interval between the respective passages of a ballistic projectile through the light curtain or beam in each of the first and second screens for calculating a speed corresponding to the measured time and the distance between the first and second screens for each ballistic projectile so measured. The calculation and display means also stores and displays the calculated speed associated with each ballistic projectile in turn.
The infra-red illumination means is comprised of a plurality of individual light sources capable of producing light in the infra-red wavelengths mounted across the lateral dimension of the screen immediately below the sun shield with an associated lens juxtaposed opposite each one of the plurality of light sources for focusing the produced light into convergent beams which overlap one another and form said light curtain. The photodetector sensor means is comprised of a focusing lens to receive the light passing through the slit in the top of the base which causes that portion of the infra-red light curtain passing through the slit to converge and be passed onto a photosensor capable of detecting perturbations in the infra-red light. Additionally, a diverging lens may be positioned between the focusing lens and the photosensor to cause the light beam to slightly diverge to encompass the lens of the photosensor.
The calculation and display means, which is responsive to said timing signals from the photodetector sensor means in the first and second screens, is also capable of causing the calculated speed to be aurally expressed by a voice modulator. Further, the calculation and display means can receive commands from an associated keyboard, link with a personal computer through a communications port, and transfer data to a printer through an infra-red port for printing reports containing the stored information.